If no water occupies soil pores, there will be no geotechnical challenges during freezing, thawing, or issues with pore ice in the soil. Engineers are interested in applied stresses (positive and negative pressures) to evaluate resistance and deformations of materials. Hence, to correctly understand the mechanics of freezing and thawing of the soil, as well as, the behavior of partially frozen soil under applied loads, it is important to first measure and understand the behavior of the pore-water pressures and their distribution within the freezing, thawing, and partially frozen soils.
When a saturated coarse-grained soil freezes, the pore space gradually fills in; hence, the soil's hydraulic and mechanical behavior change due to the phase change of water into ice within these voids. The ice matrix increases the apparent cohesion and tensile strength, and reduces compressibility and hydraulic conductivity of the soil and therefore the soil's behavior becomes more like a cemented soil or rock.
There are several challenges in measuring pore-water pressure in partially frozen soils, including: 1. Low hydraulic conductivity in the partially frozen state; 2. Continuity and availability of the unfrozen water phase; 3. Thermal effects; 4. Existence of two solid phases (ice and soil grains; 5. Time and temperature dependency of stiffness and volume of the ice matrix; and 6. Pressure melting of the ice phase under increase of effective stress or pore-water pressure and ice formation under decrease of pore-water pressure in super cooled pore-water, or decrease of temperature, or salinity of pore-water.
Increasing demands for exploiting energy and mining resources in cold climates and transportation demands in climates such as Northern Canada, need reliable engineering theories and techniques to provide safe, economical design for infrastructures required by these projects. Further, economical extraction of resources and their transport require improved engineering to reduce the costs. In addition, potential warming associated with climate change contributes to deepening active layer which may increase geohazards risks: avalanches; earth slides, falls and flows; and deglaciations. About 50% of the land in Canada is underlain with permafrost, and global warming may therefore be a security issue because of the potential geohazards that may affect the environment as well as engineering projects (highways, pipelines, railways, infrastructure, and forests). Most of the rest of Canada is underlain by seasonally freezing and thawing ground and may be affected by frost heave, thaw weakening, and thaw settlement. Freezing and thawing induced deformations affect the serviceability and durability of highways and railroads; chilled gas pipelines; oil pipelines; municipality and communication infrastructures; and all other engineering structures in areas with cold climates. Measuring pore-water pressures in partially frozen soils is of paramount importance in effective analysis and design for such applications.
The cost associated with damages induced by freezing and thawing is high. Hence, there is a need to focus on practical approaches to minimize these new and ongoing costs by determining the pore-water pressure in such partially frozen soil.
Measuring pore-water pressures in freezing and thawing soils is also of practical significance in engineering artificial ground freezing for controlling groundwater seepage and contamination transport, structural support of deep and shallow excavation, and liquid natural gas (LNG) storage.
Modern soil mechanics was born with the advent of the concept of effective stress analysis and design. Conducting effective stress analysis using effective stress material properties provides a more realistic representation of the field behavior of the ground and leads to more accurate, safer, and cost effective designs. To evaluate effective stresses and effective stress material properties in partially frozen soil, measuring pore-water pressure distribution within these soils is required. Measuring pore-water pressure distribution is also required to measure hydraulic conductivity and hydraulic gradients in these soils. Measuring pore-water pressures in partially frozen ground is more complicated than in thawed soil as phase change (ice melting or formation) occurs due to time dependent heat and mass transfer. Further, pressure melting of ice and formation of ice from super cooled water when pressure is reduced, increase the complexity of the behavior of partially frozen soils. These processes occur near freezing temperature of pore-fluid, for example 0° C. for water, and within the pressure range of interest for engineers. They result in viscous deformations, anisotropy, heterogeneity, and damage mechanisms that influence the deformation and strength response of the partially frozen soils.
The major obstacles in setting up the effective stress concept in partially frozen soils have been the difficulty of conducting reliable, accurate measurements of unfrozen pore-water pressures in partially frozen soils.
Therefore, ‘creep deformation constitutive models’ and ‘creep failure criterions’ have been historically used for analysis of the behavior of soils at subfreezing temperatures. Conducting creep tests to seamlessly simulate field conditions is time consuming and expensive. Further, the existing creep methods do not consider pore pressure generation and dissipation and do not consider effective stresses. In reality, ‘effective stresses’ and ‘effective stress material properties’ control the resistance and deformation of the soil masses and therefore the existing creep methods are not an effective way for analysis of resistance and deformation of partially frozen soils.
A saturated soil consists of soil grains and water. If there are no soil grains, water will carry the entire load (like a ship floating on water). If there is no water, the soil skeleton will carry the entire applied load. Pore pressure equations evaluate what portion of the load will be carried by the water phase when water is present and has not escaped from the voids between the soil grains. Knowledge of the pore-water pressure is needed to assess the ‘flow’ of water through a porous material and to predict the ‘effective stress’ that controls its resistance and deformation. Therefore, measurement of, and having methods for estimating the pore-water pressure response to the applied loads are desirable. Piezometers were developed to measure these pore-water pressures. Pore pressure equations, on the other hand, provide the estimation methods. The previously developed piezometers were developed for applications in thawed soils and lack the essential requirements for measuring pore-water pressures in partially frozen soils.
The term ‘hydraulic fluid’ is used in mechanical engineering and refers to the medium through which power is transferred. Hydraulic machines work more efficiently if the hydraulic fluid has ‘low compressibility’. Other major properties of the hydraulic fluid for power transfer and control are ‘fast air release’, ‘low foaming tendency’, ‘low air entrainment’, ‘high lubricating properties’, and ‘low total compressibility’. It is also desirable that the hydraulic fluid should not chemically react with, or otherwise alter, the medium that it is contained in. Air in a liquid can be in the form of ‘dissolved air’ or ‘entrained air’ (air bubbles of various sizes and foam). Even a small volume ‘air bubble’ present in the fluid can radically alter the compressibility of the fluid. Free air that can have independent pressure of the liquid is treated as an independent phase. In previously developed closed-system piezometers, the pore-pressures are transferred to a transducer in the piezometer through a fluid (which is referred to as “piezometer fluid”, which generally has been water) and through a filter. In open-system piezometers the pore-fluid in the soil just enters inside the piezometer (for example in a standpipe) through a filter. In these previous designs, the filter has been used as an interface between the piezometer and the soil and the piezometer design has been based on the concept that the piezometer fluid is behind this interface.
If there is no air in the liquid, the compressibility of the liquid is equal to its pure substance compressibility (primary compressibility). In contact with air, some of the air dissolves into the liquid or comes out of solution due to thermal or pressure variations; increasing the compressibility of the liquid (added secondary compressibility). However, at any specific pressure and temperature, only a limited amount of air can dissolve in a liquid. Therefore, the volume change of the part of the entrained air that remains as air bubbles in the liquid follows the gas law when subjected to pressure or temperature changes. The pressure and temperature in the air bubbles are in (or will reach) equilibrium with that in the liquid and the volume change of these air bubbles add to the compressibility of the liquid (added tertiary compressibility).
In a saturated soil, some of the ‘liquid water’ turns into ‘water vapor’ during the stress wave transition period of an unloading scenario. Further, dissolved air in water may come out of solution (release) when its pressure is reduced. Water vapor and the released air can create a ‘gas bubble’. The result will be a more “compressible” fluid. Further, this ‘gas bubble’ reduces the hydraulic conductivity of the soil for fluid flow and hence volume transfer of water required to transfer the new stress state is delayed. The water vapor bubbles, or water vapor in humid air bubbles, may partially turn into liquid water after the wave transition period. The amount of water vapor as ‘gas bubble’ in pore-water depends on the vapor pressure of water, which is a thermodynamic property (depends on temperature and pressure). During loading, more of the water vapor turns into liquid water and hence increases the rigidity of the liquid, as well as, hydraulic conductivity of the soil for water flow leading to faster pressure equalization. Cavitation in water in the form of ‘water vapor bubbles’ can be observed with the naked eye when water pressure approaches approximately −90 kPa. In unsaturated soils, the volume of the air bubbles increases with a reduction of the stress (in a manner similar to ideal gas law), hence it reduces the hydraulic conductivity of the soil and increases pressure equalization time.
Traditionally, piezometers with filters have been used to measure pore-water pressures. Based on the classic literature, the roles of filters in piezometers are to: 1. Separate the pore pressure from the total pressure in a soil, by transmitting pressure to the transducer only through the fluid in the measuring system; 2. Maintain the rigidity of the piezometer by eliminating (or minimizing) air from entering the measuring system. In some piezometer systems, flushing of the piezometer fluid is required before each reading; and 3. Independently measuring pore-air and pore-water pressures in an unsaturated soil, only when air and water pressures differ significantly. Generally, “low air entry” and “high air entry” filters are used to independently measure pore-air and pore-water pressures, respectively.
Continuity and hydraulic connection between the pore-water in the soil and the fluid that transmits pressure to the sensing element (transducer) in the piezometer is necessary to measure pore-water pressures correctly.
Even a high air entry filter does not prevent diffusion of air into the piezometer fluid, and the diffused air is in the form of “gas in solution”. Gas in solution may be released (because of agitation or increase in temperature) and coalesce into ‘gas bubbles’. These air bubbles accumulate and get trapped behind the filter (and within the piezometer system) and introduce error in the measured pore-water pressures.
Use of filter can delay and alter the pore pressure response of the piezometer, reduce its reliability, increase its flexibility, and cause loss of hydraulic connection between the piezometer fluid and the pore fluid in partially frozen soils. A flexible piezometer softens the pore fluid phase and hence alters the pressure being measured. Furthermore, in partially frozen soils where a small volume of unfrozen pore-water is available to transfer the pressures, rigid piezometers are required because they need only a very small volume of liquid to transfer the pressures.